Highly efficient, mild, bromide-free and acetic acid-free dioxygen oxidation of p-nitrotoluene to p-nitrobenzoic acid with metal phthalocyanine catalysts

Article abstract of DOI:10.1021/op049810b

Four metal tetracarboxyl phthalocyanines were synthesized and characterized by elemental analysis and mass spectrometry. p-Nitrobenzoic acid was efficiently prepared in high yield from bromide-free and acetic acid-free aerobic oxidation of p-nitrotoluene using metal phthalocyanines as catalysts under mild conditions in alkali-methanol solution. Up to 88.8% isolated yield of p-nitrobenzoic acid was obtained with the catalysis of tetracarboxyl phthalocyanine cobalt (0.13 mol %, based on the moles of p-nitrotoluene) optionally combined with a small amount of dimethylformamide in the presence of 2.0 MPa dioxygen at 30-60°C. The effect on catalytic performance of a carboxyl group introduced into the phthalocyanine ring was further discussed on the basis of metal coordination chemistry theory.

Full text of DOI:10.1021/op049810b

Organic Process Research & Development 2005, 9, 297−301
Highly Efficient, Mild, Bromide-Free and Acetic Acid-Free Dioxygen Oxidation
of p-Nitrotoluene to p-Nitrobenzoic Acid with Metal Phthalocyanine Catalysts
Xufeng Song,‡ Yuanbin She,*,†,‡ Hongbing Ji,§ and Yanhui Zhang
School of Chemistry and Chemical Engineering, Hunan UniVersity, Changsha 410082, P.R. China, Institute of Green
Chemistry, Beijing UniVersity of Technology, Beijing 100022, P.R. China, School of Chemical and Energy Engineering,
South China UniVersity of Technology, Guangzhou 510640, P.R. China, and College of Life Science and Bio-engineering,
Beijing UniVersity of Technology, Beijing 100022, P. R. China
Abstract:
as acetic acid are unavoidable, especially when strong
electron-withdrawing substituents such as the nitro group are
introduced into alkyl benzenes, even under high reaction
temperature and high pressure of air/dioxygen. However, the
bromide ion or acetic acid may bring undesirable side
reactions and causticity on the devices via bromination or
decarboxylation.6 Alternatively, although replacement of
conventional oxidants with H2O2 has shown unexampled
advantages in certain aspects compared with molecular
oxygen, such as reaction activity, three industrial wastes, and
environmental protection, H2O2 will self-degrade acutely and
invert into inert O2, which has to be reactivated, in the
presence of transitional metal ions or complexes thereof at
relatively high cost. Sawatari and coauthors1 developed an
efficient bromide-free catalytic system for the oxidation of
nitrotoluenes. Yet, an expensive carbon radical-producing
catalyst such as N-hydroxylphthalimide (NHPI), cobalt salt,
manganese salt, and acetic acid was used, and a relatively
high temperature such as 130 °C was adopted.
Four metal tetracarboxyl phthalocyanines were synthesized and
characterized by elemental analysis and mass spectrometry.
p-Nitrobenzoic acid was efficiently prepared in high yield from
bromide-free and acetic acid-free aerobic oxidation of p-
nitrotoluene using metal phthalocyanines as catalysts under
mild conditions in alkali-methanol solution. Up to 88.8%
isolated yield of p-nitrobenzoic acid was obtained with the
catalysis of tetracarboxyl phthalocyanine cobalt (0.13 mol %,
based on the moles of p-nitrotoluene) optionally combined with
a small amount of dimethylformamide in the presence of 2.0
MPa dioxygen at 30-60 °C. The effect on catalytic performance
of a carboxyl group introduced into the phthalocyanine ring
was further discussed on the basis of metal coordination
chemistry theory.
Introduction
p-Nitrobenzoic acid is of great importance as an essential
intermediate of pharmaceutical, organic synthesis, perfumery,
pigment, antioxidant for grease lubrication, metal antirust
agents, and the like. Conventionally, for oxidation of alkyl
benzenes, stoichiometric oxidants such as nitric acid, potas-
sium permanganate, potassium dichromate, sodium hy-
pochlorite, or organic peroxides are necessary for effective
conversion. However, these hazardous and/or toxic oxidants
generate copious amounts of wastes. Therefore, using a green
oxidant such as molecular oxygen1,2 or ozone3-5 is gathering
much attention according to the principle of green chemistry.
It is well-known that molecular oxygen could hardly be
activated in mild conditions. As a result, catalysts such as
cobalt bromide and manganese salt, as well as solvents such
In our endeavor to research toward oxidation using
biomimetic catalysts such as metal phthalocyanines and metal
porphyrins,7-10 many oxidations could be achieved under
mild conditions compared with the conventional ones.
Although Chandalia and Mukhopadhyay2 reported the mod-
erate yield of p-nitrobenzoic acid catalyzed by cobalt
phthalocyanine, the yield was not satisfactory. In the present
contribution, the above reaction with different catalysts was
systematically investigated in detail, and the significant
enhancement of catalytic activity has been achieved by
introducing a carboxyl group into the phthalocyanine rings
and by adding a cosolvent.
Results and Discussions
In our investigation, aerobic oxidation of p-nitrotoluene
to the corresponding p-nitrobenzoic acid was catalyzed by
different metal phthalocyanine catalysts. Methanol, as an
alternative solvent, instead of conventional solvents such as
* Corresponding author: Prof. Yuanbin She. E-mail: sheyb@bjut.edu.cn.
Telephone and fax: +86-10-67392695.
† School of Chemistry and Chemical Engineering, Hunan University.
‡ Institute of Green Chemistry, Beijing University of Technology.
§ School of Chemical and Energy Engineering, South China University of
Technology.
College of Life Science and Bio-engineering, Beijing University of
Technology.
(6) Zhang, S.; Xin, Z. Preparation Handbook of Fine Organic Chemical
Engineering (Chinese); Beijing: Science and Technology Reference Press,
2000; pp 349-352.
(7) Yuan, Y.; Ji, H. B.; Chen, Y. X.; Han, Y.; Song, X. F.; She, Y. B.; Zhong,
R. G. Org. Process Res. DeV. 2004, 8, 418-420.
(8) She, Y. B.; Fan, L. L.; Zhang, Y. H.; Song, X. F. J. Chem. Ind. Eng.
(Chinese) 2004, 55, 2032-2037.
(9) Zhang, Y. H.; She, Y. B.; Zhong, R. G.; Zhou, X. T.; Ji, H. B. Acta Chim.
Sin. (Chinese), 2004, 62, 2228-2232.
(1) Sawatari, N.; Sakaguchi, S.; Ishii, Y. Tetrahedron Lett. 2003, 44, 2053-
2056.
(2) Chandalia, S. B.; Mukhopadhyay, S. Org. Process Res. DeV. 1999, 3, 109-
113.
(3) Galstyan, A. G.; Tyupalo, N. F.; Andreev, P. Y. Kinet. Catal. 2003, 44,
82-85.
(4) Galstyan, A. G.; Tyupalo, N. F. Pet. Chem. 2002, 42, 314-317.
(5) Galstyan, A. G.; Tyupalo, N. F.; Patapenko, E. V.; Andreev, P. Y. Russ. J.
Appl. Chem. 2001, 74, 1777-1779.
(10) Song, X. F.; Ji, H. B.; Zhou, X. T.; She, Y. B. Fine Chem. (Chinese) 2004,
21 (6), 474-478.
10.1021/op049810b CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/05/2005
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Scheme 1
Furthermore, an electron-withdrawing substituent such as
a carboxyl group was introduced into the phthalocyanine
rings, and the effect on the catalytic performance of metal
phthalocyanines was investigated, respectively. By compari-
son with those functionalized phthalocyanines, cobalt-,
copper-, and zinc tetracarboxyl phthalocyanines exhibited an
improved yield of p-nitrobenzoic acid (entries 1-6). How-
ever, for iron phthalocyanine catalyst, its functionalized iron
tetracarboxyl phthalocyanine was less effective. To gain an
insight into the reason, the solubility of metal phthalocyanines
might be partly responsible for the yield enhancement, as
the introduction of a carboxyl group into metal phthalocya-
nines increased their solubility in the NaOH-methanol
solution in the order as cobalt > zinc > copper > iron, thus
improving the probability between substrates and catalysts.
Moreover, the introduced electron-withdrawing carboxyl
groups enhanced the d orbital splitting energy of central metal
ions, in turn, optimizing the electron configuration and
changing the coplanarity of molecular planar structure, thus
stabilizing those functionalized phthalocyanines and enabling
few dimers, which obviously decreased its catalytic perfor-
mance, formed among catalysts. Additionally, the delocal-
izing of the d orbit electrons increased the coordination
capacity between central metal ions and substrate, which
might explain the yield enhancement catalyzed by cobalt-,
copper-, and zinc tetracarboxyl phthalocyanines.
Table 1. Aerobic oxidation of p-nitrotoluene catalyzed by
metal phthalocyaninesa
p-nitrobenzoic acid
entry
catalyst
TON
yield (%)b
1
2
3
4
5
6
7
8
Ia
IIa
Ib
IIb
Ic
IIc
Id
569
589
556
570
549
569
560
549
73.9
76.6c
72.3
74.1
71.1
74.0
72.9
71.3
IId
The reason for the decreased performance of iron phtha-
locyanine could be found from the Jahn-Teller effect, i.e.,
according to crystal field theory. Octahedron structures of
metal phthalocyanine complexes got deformed more or less
with the exception of Zn2+ (d10), and forming four short
bonds was proved more stable than forming two short bonds;
thus, the two poles of octahedron were generally elongated.
Considering the metal phthalocyanines as above, first, for
cobalt, the introduction of carboxyl group changed the d orbit
a Reaction conditions: p-nitrotoluene (20 mmol), catalyst (0.13 mol %, based
on the initial amount of p-nitrotoluene), methanol (50 mL), NaOH (120 mmol),
oxygen (2.0 MPa), 12 h, 55 °C. b The isolated yield of p-nitrobenzoic acid.
c Typical resulting mixture of reaction included 3.2% p-nitrobenzaldehyde, 7.8%
p-nitrotoluene, and 1.2% p-nitrobenzyl alcohol by relative content with analysis
of HPLC besides NaOH and the solvent.
acetic acid, was introduced into the oxidation as represented
in Scheme 1, together with sodium hydroxide.
The oxidation reaction of p-nitrotoluene with 2.0 MPa of
dioxygen at 55 °C was investigated with the above-mentioned
metal phthalocyanines including Co, Cu, Zn, and Fe central
metal ions as catalysts in NaOH-methanol solution, and the
results are shown in Table 1.
It is encouraging that excellent catalytic performance of
all metal phthalocyanines were shown from aerobic oxidation
of p-nitrotoluene to p-nitrobenzoic acid with the respective
isolated yields of product more than 70%, which is much
higher than the best yield (less than 40%) reported by
Chandalia and Mukhopadhyay.2
Note: There will be a potential hazard of explosion when
using methanol, especially refluxing methanol, in a pure
oxygen environment, although the explosion-limit range of
methanol (6.0-36.5 vol %) is relatively narrower and the
flash point of methanol (9 °C) is relatively higher than that
of hydrocarbon solvents (<0 °C). Thus, much attention and
careful designs must be given more than ever, considering
the safety issues in industrial practice. For example, selecting
the reaction temperature or O2 pressure to be as low as
possible will be completely out of the explosion-limit range
of methanol (the fraction of methanol was 1.9-3.5 vol %,
and methanol was not refluxed in the present process).
energy level from t2g5eg to t2g6eg , which will make the
transition of an electron in eg orbit to t2g orbit, in turn,
decreasing the electron cloud density of the x-y planar
coordinate. As a result, its crystal field stabilization energy
(CFSE) equaled 1.0∆0. Therefore, the phthalocyanine ring
ligand was closer to the central Co2+, and the shorter N-Co
bonds favored the access of the substrate to the orienting
Co2+ (entries 1, 2). However, for iron, the effect was
unfavorable, as the introduction of the carboxyl group
2
1
2
0
changed the electron configuration from t2g4eg to t2g6eg ,
which was fully filled and fully blank, with its CFSE equaling
to 2.0∆0; thus, the improved stabilization prevented the
access of substrate and dioxygen to the central Fe2+ from
two axis-oriented sides to some extent, and decreased its
catalytic performance as a biomimetic catalyst (entries 7, 8).
For copper and zinc, since no change occurred to Cu (d9)
4
t2g6eg3 and Zn (d10) t2g6eg , the electron-withdrawing carboxyl
group contributed only to the increasing solubility of these
catalysts.
On the basis of literature and our earlier research work,
the three possible mechanisms and paths of the above
reaction could be described in eqs 1-3 of Scheme 2.
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Scheme 2
According to our earlier research results,11 the above three
Scheme 3
possible reactive paths in Scheme 2 could be effectively
controlled by using suitable conditions. The research results
showed that the above reaction only took place when a large
amount of strong alkali such as NaOH-methanol existed in
the reaction system; the yield of p-nitrobenzoic acid increased
with the increasing NaOH concentration, and much less
p-nitrobenzoic acid was obtained only where there was no
NaOH in the reactive system. Obviously, eq 2 was not the
main reactive path. Furthermore, the effects of the various
reaction conditions on the yield of the target product were
systemically investigated, which involved the initial con-
centration of p-nitrotoluene and NaOH, reaction temperature,
dioxygen pressure, and the reaction time. The results showed
that side reaction 3 could be completely controlled by
adjusting the initial concentration of p-nitrotoluene and
NaOH and selecting the proper temperature and oxygen
pressure; byproducts such as 1,2-bis(4-nitrophenyl)ethane and
1,2-bis(4-nitrophenyl)ethylene were reduced dramatically,
even null, under the recommended reaction conditions.
Therefore, eq 1 should be the main path in the above basic
liquid-phase medium, and side reactions in eq 3 hardly took
place in optimum conditions. No other explosive byproducts
such as dinitrobenzyls or dinitrosostilbene were detected
through HPLC under the optimized reaction conditions.
Further, it is strikingly surprising to find that a much better
p-nitrobenzoic acid yield could be achieved via dissolving
p-nitrotoluene into a small amount of dimethylformamide
(DMF) before reaction, as shown in Scheme 3.
In comparison with the yield of 76.6% catalyzed by cobalt
tetracarboxyl phthalocyanines without the addition of DMF
before reaction, the addition of 2 mL of DMF contributed
to the significant enhancement of the yield by about 12.2%,
which made this process promising industrially. This 88.8%
yield of p-nitrobenzoic acid was comparable with the yields
of the following recent researches. For examples, Sawatari
and coauthors1 reported that oxidation of p-nitrotoluene under
1.0 MPa air in the presence of NAPI combined with Co-
(OAc)2 (0.5 mol %) and Mn(OAc)2 (0.05 mol %) at 130 °C
afforded p-nitrobenzoic acid in 81% yield. Galstyan and
(11) She, Y. B.; Zhang, S. F.; Yang, J. Z. J Dalian UniV. Technol. (Chinese)
1999, 39, 56-59.
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299

Scheme 4
coauthors3-5 reported a closed process cycle by oxidation
of p-nitrotoluene with an ozone-air mixture in the presence
of CoBr2 at 100 °C, and the best yield reached 96.6%;
Chandalia and Mukhopadhyay2 reported that less than 40%
yield of p-nitrobenzoic acid, at 28 °C and 1.6 MPa air
pressure, was achieved in 18 h using cobalt phthalocyanine.
It should be noted that this present process has been relatively
valuable in industry, as it uses a bromide-free and acetic acid-
free reaction system in the presence of molecular oxygen at
55 °C.
Considering the effect of adding DMF, the addition of
DMF improved the solubility of cobalt tetracarboxyl phtha-
locyanines and made it more dispersible in the system. On
the other hand, since DMF was of strong polarity, the
accessible tendency between cobalt ions and DMF prevented
the dimerization of the phthalocyanine catalyst.
condenser upon the autoclave to reflux solvent while
releasing waste air during the reaction.
Experimental Section
Preparation of Metal Tetracarboxyl Phthalocyanine.
The preparations of Co, Cu, Zn or Fe tetracarboxyl phtha-
locyanine were carried out in the same process, as shown in
Scheme 4.
1. Cobalt Tetracarboxyl Phthalocyanine. To a 500-mL
three-neckflask equipped with a magnetic stirrer and reflux
condenser was added a mixture of trimellite anhydride (33.0
g, 187 mmol), CoSO4‚7H2O (13.5 g, 50 mmol), NH4Cl (4.5
g, 86 mmol), ammonium molybdic acid (0.5 g, 3 mmol),
and sufficient finely ground urea in 35 mL of nitrobenzene;
this resultant mixture was stirred for 3.5 h at 185 °C. The
obtained mixture was then washed with methanol until no
nitrobenzene could be detected, and a black-blue solid cobalt
tetraformamido phthalocyanine was afforded. Followed by
boiling about 3 min in the saturated sodium chloride solution
and filtration, the filter cake was moved into 2.0 M NaOH
solution saturated with NaCl in a 500-mL three-neck flask,
and the reaction was performed at 90 °C for 10 h until no
ammonia exited. The resulting solution diluted with ap-
propriate amount of deioned water was brought to pH < 3
with concentrated hydrochloric acid; then centrifugation and
filtration were executed, followed with dissolving well with
0.5 M NaOH solution and filtrating. After being repeated
three times, the resulting solid was washed first with water
and second with methanol until neutrality occurred. A claret
solid with metallic luster (13.4 g, 34.8%) was obtained via
drying in a vacuum. Anal. Calcd for CoC36H16N8O8: C, 57.8;
H, 2.14; N, 14.99; Co, 7.90. Found: C, 57.8; H, 2.50; N,
15.12; Co, 7.50. Anal. HRMS (M+): Calcd for CoC36H16N8O8,
747.0442; Found, 747.0442; m/z 747 [M + H]+. IR(KBr) ν
cm-1: 3387.2, 1683.9, 1614.9, 1520.9, 1487.1, 1396.8,
1331.9, 1247.7, 1149.9, 1090.3, 944.0, 847.2, 779.4, 741.3,
644.4, 557.7, 484.3, 434.0.
Conclusions
In conclusion, the isolated yield of p-nitrobenzoic acid
catalyzed by cobalt tetracarboxyl phthalocyanine from the
oxidation of p-nitrotoluene could be provided up to 88.8%,
together with a small amount of dimethylformamide, under
2.0 MPa O2 at 55 °C in the presence of molecular oxygen.
Since methanol was not refluxed and the reaction temperature
and O2 pressure in the present process were so low, the partial
pressure of methanol was out of its explosion-limit range.
At the same time, no other explosive byproducts such as
dinitrobenzyl or dinitrosostilbene were detected by HPLC
under the optimized reaction conditions. The novel technol-
ogy should be much safer than the conventional processes.
Furthermore, the process was environmentally friendly,
economical, and anticorrosive for equipment compared to
the conventional methods, and the process should find
practical application in industry.
In general, however, there might be certain potential
hazards that should be paid more attention when a reaction
was carried out in pure oxygen systems. It would be, of
course, more preferable and safer if the pure oxygen were
to be replaced with air or oxygen-rich nitrogen, and the
reaction temperature and O2 pressure were chosen to be as
low as possible. The safer process and technology can be
realized by some modification of the present novel technol-
ogy for using air instead of dioxygen and adding a reflux
2. Copper Tetracarboxyl Phthalocyanine. The same
process as above was carried out except 12.0 g of CuSO4‚
5H2O was used instead of 13.5 g of CoSO4‚7H2O. 8.9 g,
and a claret solid with metallic luster (8.9 g, 23.0%) was
given. Anal. Calcd for CuC36H16N8O8: C, 57.45; H, 2.13;
N, 14.89; Cu, 8.44; Found: C, 57.01; H, 2.54; N, 14.84;
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Cu, 7.76. Anal. HRMS (M+): Calcd for CuC36H16N8O8,
Found, 752.0168; m/z 747 [M + H]+. IR(KBr) ν cm-1:
3354.2, 1659.0, 1613.4, 1574.4, 1506.0, 1384.2, 1327.1,
1245.9, 1147.3, 1087.5, 931.8, 845.4, 770.3, 736.6, 660.0,
549.0, 476.8, 430.0.
3. Zinc Tetracarboxyl Phthalocyanine. The same
process as above was carried out except 6.5 g of ZnCl2 was
used instead of 13.5 g of CoSO4‚7H2O, and a bottle-
green solid (1.2 g, 3.1%) was given. Anal. Calcd for
ZnC36H16N8O8: C, 57.29; H, 2.12; N, 14.85; Zn, 8.67;
Found: C, 57.01; H, 2.54; N, 14.84; Zn, 7.76. Anal. HRMS
(M+): Calcd for ZnC36H16N8O8, Found, 754.0481; m/z 747
[M + H]+. IR (KBr) ν cm-1: 3396.6, 1702.4, 1614.2, 1578.2,
1523.5, 1419.6, 1343.0, 1281.7, 1250.1, 1049.7, 928.2, 861.4,
832.2, 766.1, 739.8, 672.4.
4. Iron Tetracarboxyl Phthalocyanine. The same
process as above was carried out except 9.5 g of FeCl2‚4H2O
was used instead of 13.5 g of CoSO4‚7H2O, and a blue-
black solid (1.0 g, 13.0%) was given. Anal. Calcd for
FeC36H16N8O8: C, 58.24; H, 2.16; N, 15.06; Fe, 7.51;
Found: C, 58.48; H, 2.78; N, 15.52; Fe, 7.42. HRMS (M+):
Calcd for FeC36H16N8O8, Found, 744.0356; m/z 744 [M +
H]+. IR (KBr) ν cm-1: 3339.3, 3181.7, 1660.6, 1611.5,
1576.8, 1513.3, 1383.9, 1328.6, 1250.2, 1150.0, 1088.0,
939.7, 850.0, 771.0, 741.0, 645.0, 432.7.
with 50 mL of methanol and 120 mmol of NaOH as well as
0.13 mol % of the above synthesized metal phthalocyanines
(based on the initial amount of p-nitrotoluene). The reaction
was carried out for 12 h after 2.0 MPa dioxygen was
introduced into the system, accompanied by magnetic stirrer
stirring. When the reaction was finished, the reactor was
allowed to cool to room temperature, and the pressure was
released. The reaction mixture was filtered with the deionized
water and acidified with hydrochloric acid to keep the pH
less than 3.0. A white solid product was obtained (2.42-
3.10 g) after filtration followed with drying.
Acknowledgment
We are indebted to the Key Project of the National Natural
Science Foundation of China (No. 20436010), the Major
Project of Science Foundation of Hunan University, the
Science and Technology New Star project of Beijing
Scientific Committee (No. 953811200), the Beijing Natural
Science Foundation (No. 2002002), and the Youth Science
Research Foundation of Beijing University of Technology
(No. 00194) for financial support of this work.
Supporting Information Available
Experimental details. This material is available free of
charge via the Internet at http://pubs.acs.org.
Preparation of Nitrobenzoic Acid from Oxidation of
Nitrotoluene with Catalysis of Metal Phthalocyanines in
the Presence of 2.0 MPa Dioxygen at 50 °C for 12 h. To
a 200-mL autoclave was added 20 mmol of p-nitrotoluene
Received for review October 14, 2004.
OP049810B
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